1. Field of the Invention
The present invention relates to an exhaust-gas purifying catalyst. More specifically, it relates to an exhaust-gas purifying catalyst which is good in terms of the catalytic ignitability.
2. Description of the Related Art
As the concerns over the environmental issues have been growing recently, the measures against exhaust gases emitted from internal combustion engines such as automobile engines have been taken seriously. It is because problems such as pollution and environmental deterioration have occurred when exhaust gases emitted from internal combustion engines are emitted into the atmosphere as they are. Hence, the exhaust gases are emitted into the atmosphere after they are purified by exhaust-gas purifying catalysts.
Exhaust-gas purifying systems using exhaust-gas purifying catalysts have been employed widely, because they can reduce the exhaust-gas emission remarkably, compared with the other measures, without applying so much load to internal combustion engines, such as automobile engines, which emit exhaust gases.
In general, exhaust-gas purifying catalysts are made in the following manner. A porous loading layer is formed on a surface of a catalyst support substrate which exhibits heat resistance. The loading layer is composed of heat-resistant inorganic oxides, such as alumina. A catalytic ingredient is loaded on the loading layer. The exhaust-gas purifying catalysts convert nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO), which are contained in exhaust gases, into unharmful nitrogen, carbon dioxide and water, respectively, by the action of catalytic ingredients.
It has been known that the exhaust-gas purifying performance of catalytic ingredients is affected strongly by temperature. In general, when purifying exhaust gases with catalytic ingredients, the purification has been carried out at a temperature of 300° C. or more. Accordingly, when the temperature of exhaust gases is low, such as immediately after starting automotive engines, the catalytic activity of catalytic ingredients is so low that it has been difficult to purify exhaust gases satisfactorily.
As a method for improving the purifying performance and ignition performance of exhaust-gas purifying catalysts, it has been known to increase the loading amount of catalytic ingredients. Moreover, as a method of upgrading the ignition performance of exhaust-gas purifying catalysts, a method is available which uses a plurality of catalyst support substrates. For example, Japanese Unexamined Patent Publication (KOKAI) No. 6-99,076 and Japanese Unexamined Patent Publication (KOKAI) No. 6-205,983 disclose such exhaust-gas purifying catalysts.
Japanese Unexamined Patent Publication (KOKAI) No. 6-99,076 discloses a metallic support catalyst. The metallic catalyst comprises two catalyst support substrates, a loading layer, and Pt and Rh. The catalyst support substrates are fastened so as to be separated at an interval. The loading layer is formed on a surface of the respective catalyst support substrates. The Pt and Rh are loaded on the loading layer in an amount of 1.0 g and 0.2 g with respect to 1 L of an apparent volume of the catalyst support substrates, respectively.
Japanese Unexamined Patent Publication (KOKAI) No. 6-205,983 discloses an exhaust-gas purifying catalyst which comprises two catalyst support substrates, a loading layer, and Pt and Rh. Similarly, the catalyst support substrates are fastened so as to be separated at an interval. The loading layer is formed on a surface of the respective catalyst support substrates. The Pt and Rh are loaded on the loading layer. However, the exhaust-gas purifying catalyst is characterized in that one of the catalytic ingredients, Pt, is loaded in a greater loading amount on an upstream side of the respective loading layers. For example, in the exhaust-gas purifying catalyst, the Pt and Rh are loaded on an upstream portion of the loading layer formed on the upstream-side catalyst support substrate in an amount of 10.0 g and 0.3 g with respect to 1 L of an apparent volume of the upstream-side catalyst support substrate, respectively; and the Pt and Rh are loaded on the other portion of the loading layer formed on the upstream-side catalyst support substrate in an amount of 1.5 g and 0.3 g with respect to 1 L of an apparent volume of the upstream-side catalyst support substrate, respectively. Moreover, the Pt and Rh are loaded on the loading layer formed on the downstream-side catalyst support substrate in an amount of 10.0 g and 0.3 g with respect to 1 L of an apparent volume the downstream-side catalyst support substrate, respectively.
Note that it has been required for conventional exhaust-gas purifying catalysts, involving the exhaust-gas purifying catalysts disclosed in the aforementioned patent publications, to exhibit furthermore enhanced performance.